1. Field of the Invention
The present invention relates to an organic electroluminescence device which includes a hole injection or transport layer between a cathode and an anode.
2. Discussion of the Related Art
Recently, the demand for a flat panel display device occupying less area is rising according to the tendency of manufacturing a wide-screen display device.
A technology for an organic electroluminescence device, which may be called an organic light emitting diode (OLED), as one of the flat panel display devices has been rapidly developed. And, various prototypes were already released.
The organic electroluminescence device is a device emitting light in a following manner. First of all, if electric charges are injected in an organic layer between a cathode and an anode, a pair of electron and hole is formed to disappear. While disappearing, the electron/hole pair emits light.
Hence, the organic electroluminescence device, which can be formed on a flexible transparent substrate like plastics, is drivable at a voltage (below 10V) lower than that of a plasma display panel (PDP) or an inorganic electroluminescence (EL) display.
The organic electroluminescence device needs relatively low power consumption and provides the excellent color sense.
In order to drive the organic electroluminescence device at low voltage, it is important to maintain very thin and uniform thickness (100˜200 nm) of the organic layer and stability of the device.
In providing high efficiency to the organic electroluminescence device, it is important to sustain density balance between holes and electrons.
For instance, if an electron transport layer (ETL) is situated between an emitting layer (EML) and a cathode, most of the electrons injected in the emitting layer from the cathode move toward the anode to be recombined with holes.
Yet, if a hole transport layer (HTL) is inserted between the anode and the emitting layer, the electrons injected in the emitting layer are blocked by an interface of the hole transport layer. If so, the electrons are unable to further proceed to the anode but remains in the emitting layer only. Hence, recombination efficiency is enhanced.
Meanwhile, a hole injection layer (HIL) is additionally inserted between the anode and the hole transport layer.
If an organic substance having a work function of 5.0˜5.2 eV is selected by considering a work function (4.7˜5.0 eV) of the anode and ionization potential (IP) of the hole transport layer, an energy barrier in hole injection from the anode to the hole transport layer is lowered to enable efficient hole injection.
A process of fabricating an organic EL device is explained as follows.
(1) First of all, an anode material is coated on a transparent substrate. ITO (indium tin oxide) is usually used as the anode material.
(2) A hole injection layer (HIL) is coated on the anode material. Copper phthalocyanine (CuPC) as the hole injection layer is mainly coated 10˜30 nm thick thereon.
(3) A hole transport layer (HTL) is introduced. (4,4′-bis[N-(1-naphthyl)-N-phenthylamino]-biphenyl(NPB) as the hole transport layer is deposited 30˜60 nm thick.
(4) An organic emitting layer is formed thereon. In doing so, a dopant is added thereto if necessary. In case of green emission, Alq3 (tris(8-hydroxy-quinolatealuminum)) as the organic emitting layer is deposited 30˜60 nm thick. And, MQD (N-Me thylquinacridone) is mainly used as the dopant.
(5) An electron transport layer (ETL) and an electron injection layer (EIL) are sequentially coated thereon or an electron injection transport layer is formed. In case of green emission, the electron injection/transport layer is not usually used since Alq3 in (4) has sufficient electron transport ability.
(6) A cathode is coated thereon. And, a protecting layer is finally formed thereon.
However, the related art device needs a material lowering the energy barrier in hole injection to effectively inject holes.